Gloucester City Council Factsheet

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					                           Gloucester City Council
                              Environmental Health
                                 Public Health: Rats

         Distribution and Habitat

         Worldwide, rodents are the largest group of mammals with over 1,500 representative
         species. Only fifteen are present in Britain. They include the squirrels, dormice and voles
         as well as the rats and mice. Being mammals, rodents are warm-blooded and give birth to
         live young which they suckle. They are distinguished from other mammals by the
         characteristics of their teeth, particularly their continuously growing incisors which are
         used for knawing.

         The commensal rodent species considered as pests in Britain are:

         •   Rattus norvegicus       -   The common, Norway, sewer or brown rat.
         •   Rattus rattus           -   The ship, roof or black rat.
         •   Mus domesticus          -   The house mouse. (See mouse factsheet).

         These species have become so well adapted to living in close association with man that
         they have acquired a world-wide distribution. Their adaptability has enabled them to
         survive extremes of climate from the frozen tundra to the dry, barren desert.

         Common Rat (Brown Rat)

         The common rat has only been recorded in Britain since early in the 18th Century. It is
         thought to have been introduced in shipping from Russia, and did not originate in Norway,
         despite one of its common names.

         It is now by far the more abundant of the two rat species and is widely distributed in both
         urban and rural areas. It occurs both indoors and outdoors away from human habitation
         and is the species often associated with sewer systems.

Gloucester City Council   Tel 01452 396396 Fax 01452 396340
Herbert Warehouse         Email
The Docks                 Minicom 01452 396161
Gloucester GL1 2EQ
Ship Rat (Black Rat)

The ship rat probably originated in south-east Asia and was thought not to have reached
western Europe until the Middle Ages, possibly returning with the Crusaders. However
remains have been discovered in Roman deposits in York and London dating back to the 3rd
and 4th centuries.

Although once the dominant rat species in this country, it is now rate and confined mainly
to port areas in Tilbury, Liverpool and Avonmouth. It may be found very occasionally in
some inland towns, especially those linked to ports by canals. In Britain it lives only
indoors and is rarely found in sewers. Its range continues to contract.

Physical Characteristics

Table 1: Physical dimensions of adult rats

                                Common Rat                       Ship Rat
Weight                          100-500g                         100-300g
                                (average 335g)                   (average 225g)
Combined head & body length     200-270mm                        150-220mm

Tail length                     150-200mm                        180-250mm

Colour is not a reliable diagnostic feature. Typically the common rat is brownish grey on
the back and grey underneath, but colour varies and black forms have been found.
Although usually black, the ship rat can also be grey, brown or tawny with white or grey

Sensory Attributes

In order to control rats and mice successfully it is necessary to consider how they interact
with their environment. One of the most important aspects of the biology of any animal is
its ability to pick up clues about its surroundings using its five senses of touch, taste,
smell, hearing and sight. Without this information the animal would be unable to respond
appropriately, and its survival would be threatened.


Touch is a very important sense in rats and mice. The young respond to touch as soon as
they are born whereas their sight does not develop until several days later.

It is likely that touch is used for hedging the shapes of objects and for the recognition of
landmarks in their environment. Hairs sensitive to touch are found all over the fur, and
particularly among the vibrissae or whiskers (Figures 2 and 3). They are used to keep rats
and mice in close contact with objects in their surroundings, and by this means they
quickly establish familiar routes in which the directions are sensed by touch as well as by

Rats and mice also have the ability to learn their surroundings by the constant practice of
sequences of muscular movements. Such an ability may be useful in learning the home
range and responding automatically to danger by unimpeded flight.

Taste is undoubtedly important in assessing the palatability of foods but it is likely that
smell and visual clues are also used. It is believed that rodents are able to distinguish the
four basic tastes of sweet, sour, bitter and sale.

It should be realised that, as with humans, there can be a great deal of variability in the
response to taste both within and between populations of commensal rodents.


Smell and taste are closely related but rats and mice do have a well developed sense of
smell and it is likely that much of the information about their surroundings is obtained in
this way. They use smell to discriminate between individuals of the same species. Indeed,
a discrete colony will be bonded together by a recognised smell and will defend a territory
against intruders which do not have the same odour.

Smell is important to colony members in communicating breeding status. There is some
evidence that female mice can distinguish their own offspring using smell alone. This
sense is also used for the location of food. Human odour does not appear to be a deterrent
for rats and mice.


Rats and mice have an acute sense of hearing. They are particularly sensitive to any
sudden noise and to high frequency sounds (ultrasound). A slight squeak, if it is high
pitched will arrest attention, and a loud noise produces instant flight. Ultrasounds are
used primarily for communication between individuals.


Rats and mice are colour blind and compared with other senses, sight seems to play only a
minor role in their lives. The main functions of the eyes appear to be in appreciating
differences in light intensities and detecting movements at close quarters. The former is a
useful adaptations for animals that are mainly nocturnal. Detection of movement may
elicit ‘attentive’ behaviour in which the rodent is alert and waiting for further sensory

Physical Attributes

The physical attributes of an animal are also very important for its success. They provide
the limits in the extent to which individual rodents and rodent populations can utilise their
habitats and exploit their surroundings.


Gnawing is part of the innate behaviour of rodents and is not necessarily associated with
the search for food or water. Almost any kind of material may be gnawed.

The extent to which a particular object such as a water pipe or the bottom of a door is
susceptible to attack is only partly dependent upon its hardness. Its shape, size and
position may also be important. This is because gnawing is only possible if the rodent can
get enough purchase on the object with its upper incisors to bring its harder, lower incisors
into play.
Gnawed material may not be swallowed, nor even necessarily tasted, which makes it very
difficult to find repellent chemical substances that will prevent this activity.

If normal wear on the continually growing incisors is prevented because the upper and
lower incisors are not directly opposed, grotesque tusk-like teeth will form. Individuals so
affected eventually die either from being unable to feed properly or from ingrown incisors.

Besides gnawing, the incisors are used for biting and holding food, fighting and the
excavation of burrows.


A burrow, or indeed any harbourage, has several functions. It acts as a place to rear
young, to rest during the daytime and to escape from predators. Rats and mice are quick
to exploit cavities in the walls, roof spaces and ducts of buildings which they use for
harbourage. In stockpiled food, they often nest in the cavities between sacks and, where
possible, in the sacks themselves.

Common rats are far more active burrowers than the other two species. The holes formed
average about 100mm in diameter, and most do not extend far into the ground.
Sometimes a complicated tunnel system with several openings is produced. Typically, such
a system can be found around the outside of buildings, in embankments, hedgerows,
rubbish tips, amongst tall undergrowth and in other similar situations. If at all possible,
rats prefer to live close to sources of both food and water.

In the absence of common rats, ship rats will sometimes burrow beneath the wall of

Climbing and Jumping

Both species, especially the ship rat, are good climbers and they can climb vertical walls if
the surfaces are sufficiently rough. Common rats can also climb up the outside of vertical
pipes (such as drainpipes) provided they are against a wall, by bracing their backs against
one of the surfaces. They can even climb the inside of a vertical pipe if its diameter is no
greater than 100mm.

Ship rats have no difficulty travelling by way of pipes, beams or cables. They have even
been known to move between the floors of buildings using lift shafts. In buildings, ship
rats are mainly found living at roof height, and almost invariably so when common rats are
also present.

Common rats can jump to a height of about 700mm and mice to 250mm. Ship rats can
jump more than one metre from a standing start.

Rats may enter buildings at ground level through small openings in the fabric and at roof
height by climbing the walls directly or with the aid of unguarded cables and external

Their climbing and jumping abilities and relatively small size enable them to range freely
in most environments and to find safe harbourage. These characteristics must be taken
fully into account before buildings can be effectively proofed.

Rats are reasonably good swimmers so open water does not necessarily hinder their
movements. Indeed, common rats are often found beside water, living on the banks of
streams and ditches and also in sewers. They dive well and can remain submerged for a
considerable time. They have also been known to enter premises from drains or sewers,
passing through the water seal (S-bend) of the lavatory bowls.

Population Development and Territorial Behaviour

Common Rat colonies typically develop from a pair or a single pregnant female. Social
relationships are harmonious within small colonies which each occupy a territory. The
territory of a rodent colony is an area that is smaller than its home range and is defended
by members of that colony. Any intruders into the territory, which is often based on
suitable breeding sites, are repelled vigorously and may even be killed if escape is
prevented. Defendable sites may be the only ones that can be used successfully for raising
young and can be the limiting factor for population numbers in a particular area.

A dominance structure develops as population density increases. High-ranking individuals
usually occupy favoured positions close to a food source. Low-ranking members of a
colony may be allowed to feed only while dominants are inactive, for example, during
daylight. Male rats will also compete to gain access to a receptive female – the stronger,
more vigorous males copulating with the female.

Such territorial and dominant behaviour can spread a rodent population. Poison bates will
need to be positioned so that they are accessible to all members of the population to
ensure effective control.

Reproduction and Life Cycle

Rodents can breed even more efficiently than rabbits and sizeable infestations can develop
very quickly from a single pair.

Conditions which suit a rapid population increase are even temperatures, surplus food with
adequate water and indisturbed cover for rearing young and escaping from enemies.
Under these optimum conditions rats and mice may breed throughout the year. In less
favourable habitats commensal rodent breeding takes place mainly in the summer and

The life cycle of rats is as follows:-

They are capable of reproducing at the age of about 3 months. Pair-bonds are not formed
as mating is carried out on an opportunistic and promiscuous basis. When a female
becomes sexually receptive, her scent attracts all the local males. The female is receptive
to the male for only a relatively short time (about 12 hours) each cycle. Mating is brief
and can take place with a number of males. After mating and conception there is a
relatively short period of pregnancy (gestation). Birth is followed by a similar time period
after which the litter is weaned.
Typical Life Cycle of a Fertile Female Rat


                                             Weaned (3 weeks old)

Dies (less than one year old)
                                                        Sexually mature
                                                        (3-4 months old)

Several litters                         oestrus                  Mates and

                                  Gives birth

There is, however, one factor which can speed up this cycle considerably. This is termed
as post-partum oestrus. Unlike many mammals, rats do not have to wait until the original
litter is weaned and the female has stopped giving milk before coming back into oestrus.
This means that the female rat may be willing to mate and can conceive again soon after
the original litter is born. This only occurs when conditions are favourable for the survival
of the young.

Commensal rodents may give birth every 24 to 28 days and this can give rise to very rapidly
increasing populations. At very high densities reproduction rates decrease to take account
of the reducing availability of resources and the increasing competition within the

Table 3: Breeding activity of commensal rodents

                        Common Rat               Ship Rat

Sexually mature         8-12 weeks               12-16 weeks
Gestation period        21-24 days               21-22 days
Litter size (average)   6-11                     6-10
Weaning period          3-4 weeks                3-4 weeks

Harbourage, Burrows and Nests

The burrows of the common rat outdoors are usually easy to recognise, and often situated
on sloping ground such as banks or the sides of ditches, or beneath some form of cover
such as logs, tree roots, flat stones or pavements. The entrance holes are generally 70 to
120mm in diameter. Earth dug from the burrow remains in a heap adjacent to the
entrance. This will be trampled if the burrow has been well used. Cobwebs at the
entrance to a burrow indicate that it is not currently in use.
One of the important functions of the mother is to provide a nest for the young. Quite
substantial nests are often made from materials such as grass, leaves, paper, sacking and
insulation. The temperature in a nest may be considerably above that of the outside. A
secure site is essential for the success of the litter. Young rodents are born blind, totally
helpless and without hair except for short vibrissae. They become fully furred at about 14
days with eyes and ears open and incisor teeth showing.

The males play little part in the rearing of the offspring but the mother is normally very
attentive. She becomes very aggressive and defends her young from outside interference.
If the nest is disturbed, the mother may move all the young to another safe location.
Despite this attention, young rodents are very vulnerable. Many die before becoming
sexually mature.

Sometimes a neat, well-defined hole will appear in the ground with little or no heaped
earth beside it. This generally indicates that a rat has emerged from below ground;
frequently there is a connection with a drain or sewer. This may occur when buildings are
demolished and the old drains have not been sealed effectively.

In many buildings, harbourages, are almost limitless due to the presence of numerous dead
spaces and undisturbed areas.

Ship rats usually find harbourage high up in a building. Common rats will tend to exploit
shelter in the fabric of the building such as in a hollow wall. However, the possibility of
them living in associated drains and sewers should not be overlooked.

Occasionally the remains of a rodent nest can be found indoors, usually in an undisturbed
area. Any readily available material can be used to make the nest. If sacking, paper or
packaging is seen to be gnawed, it can often indicate that nests are being made. This is an
important point to note during the survey of an infestation.

Impact of Rodents on Human Populations


The commensal rodents are of particular public health importance because of their close
association with human settlements, their worldwide distribution, and the link they can
provide between man and other carriers of infection.

Weil’s Disease

There is a type of leptospirosis associated with rats called Weil’s disease (also known as
leptospiral jaundice).       The casual organism is the bacterium Leptospira
icterohaemorrhagiae which has been found in the kidneys of up to 50% of the individuals in
sampled rat populations. It is shed in the urine of infected rats which do not appear to
suffer harm.

Wet environments are necessary for the survival of the bacterium outside the rat’s body.
It is usually transmitted to humans by contact with contaminated water or moist soil, or by
direct contact with rats. The excreted organism enters the human body through cuts and
abrasions, or through the mucous membrane of the nose and mouth. Weil’s disease used
to occur most commonly among agricultural workers and those who worked in damp places
such as sewers and abattoirs. More recently, there has been a rise in the number of cases
amongst those engaged in recreational water activities (swimmers, water-skiers and
Weil’s disease has been a notifiable disease for some years, and can range in severity from
mild flu-like symptoms to jaundice, renal failure and death. Public education is very
important in its prevention, and the issue of ‘warning cards’ to those particularly at risk
(and this certainly includes those carrying out rodent control) should be encouraged. The
Health and Safety Executive have published a warning card entitled ‘Leptospirosis’. (See
Section on Legislation).


Another disease of practical significance in this country is salmonellosis caused by bacteria
of the Salmonella group. It ranks as one of the most widespread of animal-borne diseases.
Infection in humans commonly occurs from the contamination of food and drink with
rodent excreta, or as a result of the passive transmission of the bacteria by rodents.
Salmonellosis is a type of food poisoning and symptoms can include acute gastroenteritis
accompanied by headache, fever and vomiting.


Of the many diseases carried by rats and mice, plague is the most infamous. It is caused
by bacterium which is transmitted to man from rats mainly by the rat flea (Xenopsylla
cheopsis). Between outbreaks in human populations the disease persists as ‘sylvatic foci’
among wild animals. These are found at present in many parts of the world including
south western USA, South America, South Africa and the USSR. Usually the ship rat is the
species involved but occasionally the common rat has been found to be an important
carrier. Plague has not been recorded in Britain since early this century, but it remains a
potential hazard – hence the care taken at ports and airports to prevent wild rodents from
being introduced into the country. (See Section on Legislation).

Other Diseases

Other rodent-borne diseases transmitted to man include rat-bite fever, lymphocytic
choriomeningitis and murine typhus. Parasites carried and transmitted by rodents include
ringworm, mites, nematodes, tapeworms, ticks and fleas. There are also a number of
diseases that can be transmitted to animals, including foot and mouth in cattle and sheet.

                                                                                     MAY 2002